The invention pertains methods of fast exploration, by a surveillance radar antenna, on 360xc2x0 in azimuth. The invention relates furthermore to surveillance radars implementing a method of fast exploration on 360xc2x0 in azimuth.
The invention can be applied especially to sea patrol radars or helicopter detection radars.
Certain missions for 360xc2x0 surveillance require fast exploration in azimuth in order to obtain several measurements on the target during the detection time. This requirement can be explained especially for the following reasons:
firstly, the target may have motions of rotation on itself. In particular, if the target consists of the blades of a helicopter, a submarine periscope or a ship""s mast. During rotation, the target presents a variable surface to a radar. This surface is commonly designated by the acronym SER (or service equivalent radar). Now to obtain radar coverage on 360xc2x0 , the approach used consists in rotating the beam of the radar antenna. Given the type of target, the faster the rotation of the radar antenna beam, the greater is the probability that the beam of the radar antenna will be directed towards the target when it has a big SER;
secondly, the target may be buried in clutter with a correlation time smaller than that of the target, for example when the clutter is caused by the sea. The target can be separated from the clutter by carrying out several measurements during the detection time.
The approach used to carry out fast exploration with a radar antenna on 360xc2x0 in azimuth consists in making the radar antenna beam rotate at high speed. To make the radar antenna beam rotate at high speed, the prior art techniques have undergone development from techniques based on automatic control systems to electronic scanning techniques.
Automatic control techniques achieve the mechanical rotation of the radar antenna by means of automatic control systems. Developments in the field of automatic control systems are used to obtain high rotation speeds. Sea surveillance radar, which is the subject of an article by J. Croney and A. Woroncow, xe2x80x9cRadar Polarization Comparisons In Sea-Clutter Suppression By Decorrelation And Constant False Alarm Rate Receiversxe2x80x9d in xe2x80x9cThe Radio and Electronic Engineerxe2x80x9d, Vol. 38, No. 4, October 1969, provides a rotational speed performance of this kind. The radar in question has two back-to-back mounted antennas with the same angular aperture. The antennas have cross polarizations that are vertical in one case and horizontal in the other. The radar power is switched over at each half-revolution, from one antenna to the other, in such a way that the p.p.i. image is divided into two 180xc2x0 parts. The image of the first part is repeated on the second part with an orthogonal polarization in order to choose the polarization, and therefore the antenna, that is most suitable as a function of the sea clutter. The antennas are protected by an aerodynamically-shaped radome. The rotation speed of the antenna may reach 600 rpm.
Speed performance characteristics of this kind call for powerful and complex automatic control systems. The drawbacks of these systems are that they reduce reliability, are bulky and heavy and generally costly.
The development of radar techniques is leading to techniques that implement electronic scanning. The electronic scanning is used for example in certain advanced technology radars for combat aircraft. However, at present, electronic scanning antennas cannot cover 360xc2x0.
One of the goals of the invention is to provide for fast exploration on 360xc2x0 in azimuth with an antenna radar that does not have the drawbacks of the above-described techniques based on automatic control systems.
To this end, an object of the invention is a method of fast exploration on 360xc2x0 in azimuth by a radar antenna comprising two elementary electronic scanning antennas, installed back to back, capable of being electronically shifted by a maximum angle xc2x1ad, the method consisting in:
driving the antenna rotationally at a speed w1,
simultaneously controlling the electronic aiming of each elementary electronic scanning antenna so that the scanning of the radar beam of the antenna is done at a speed w2 on one rotation, with w2 verifying the following relationship:       w    2    =            w      1        xc3x97                  2        ⁢        π                    2        ⁢                  xe2x80x83                ⁢                  (                                    π              2                        -                          α              d                                )                    
switching over the microwave signal sent, alternatively between the two elementary electronic scanning antennas, at a determined rate,
before each rotation i of the radar beam, initializing the electronic aiming of each elementary electronic scanning antenna at a determined angle xcex8i.
The selection-switching rate is a function of the frequency of recurrence of the radar transmissions, the mechanical rotation speed, the aperture of the pencil beam of each antenna and the maximum aim shifting angle xcex1d. To obtain an even illumination in azimuth for each elementary antenna, the selection-switching period is smaller than the time of illumination of a target.
A first preferred embodiment is characterized by a selection-switching rate identical to the frequency of the radar transmissions.
A second embodiment is characterized by a selection-switching rate that is variable as a function of the aiming direction.
A third embodiment is characterized by the association of the selection-switching control with polarization control. To each radar transmission, there corresponds a specified polarization.
The method can advantageously be used to make the radar beam rotate at a speed greater than the speed of mechanical rotation of the antenna.
Depending on its embodiments, the method is advantageously used to match the selection-switching rate to the direction of aim and choose the type of polarization at each radar transmission.
In one particular embodiment, the method can be used to obtain a rotation speed of the radar beam that is variable while having a constant mechanical rotation speed.
An object of the invention furthermore is a radar implementing the method. The radar comprises an antenna, an automatic control system, an aim computer and a selection-switching means.
The automatic control system drives the antenna in slow rotation at a speed w1.
The antenna has two back-to-back mounted elementary electronic scanning antennas that can be shifted by a maximum angle xc2x1xcex1d.
The aim computer simultaneously controls the electronic aiming of each elementary electronic scanning antenna so that the scanning of the radar beam of the antenna is done at a speed w2 on a rotation, w2 verifying the relationship:       w    2    =            w      1        xc3x97                  2        ⁢        π                    2        ⁢                  xe2x80x83                ⁢                  (                                    π              2                        -                          α              d                                )                    
and so that, before each rotation i of the antenna radar beam, the electronic scanning of each elementary electronic scanning antenna is initialized at a determined angle xcex8i.
The selection-switching means switch the microwave signal over alternatively between the two elementary electronic scanning antennas. The selection-switching means typically comprise a microwave selection switch driven by a selection switch control. The selection switch control is a function of the frequency of recurrence of the radar transmissions, the mechanical rotation speed and the aperture of the pencil beam of each antenna. A relay switches over the microwave transmission signal to one of elementary electronic scanning antennas.
In a first particular embodiment, the antenna has two reflector arrays.
In a second particular embodiment, the antenna has two active antennas.
The radar has the advantage of not requiring any complex and costly automatic control system.